A hybrid electric vehicle (HEV) utilizes multiple sources of energy in order to improve fuel economy while reducing certain vehicle emissions. Typically, an HEV includes a rechargeable energy storage system (ESS), usually a battery or battery pack having a relatively high energy density, with the ESS being electrically connected to at least one of the multiple energy sources. The multiple energy sources most often include a gasoline or diesel internal combustion engine and at least one electric motor/generator. However, other HEV designs may alternately employ a fuel cell and/or another power source in place of the internal combustion engine in order to further reduce vehicle emissions.
In a typical “mild” hybrid design in particular, an energy conversion system, which is usually configured an internal combustion engine, provides the power necessary for propelling the HEV, with the engine being configured to shut off when the mild HEV is idle or at a standstill. In this manner fuel is conserved, particularly during stop-and-go traffic conditions. When a driver depresses an accelerator pedal to launch a mild HEV, an electric drive motor connected to an auxiliary battery provides an initial burst of power lasting through an interval required for cranking and starting the engine. The drive motor is not used to power the vehicle independently of the engine, as would a conventional or “full” HEV. However, a mild HEV may still be configured to provide regenerative braking and/or idle stop capabilities.
Regenerative braking is used on certain mild HEV as a means of capturing braking energy to further optimize fuel economy. A controller can select a first rotational direction of the electric motor/generator as needed to allow that device to act as a generator during a braking event. Acting as a generator, the ESS can be recharged while a negative torque is applied to the road wheels, thus slowing the HEV. Likewise, during normal driving operations the controller can select a second rotational direction of the electric motor/generator to allow that device to act as an electric motor for powering various systems aboard the HEV in conjunction with the ESS. Because regenerative braking improves overall fuel economy, it can be desirable to maximize both the magnitude and the duration of the regenerative braking during a given regenerative braking event.
In an automatic transmission aboard an HEV, a hydrodynamic torque converter assembly can provide a variable slip fluid coupling between an input shaft of the transmission and the power source or sources. Within certain torque converter assemblies, a lockup-style torque converter clutch (TCC) can be selectively engaged above a threshold speed, usually around 30 to 35 miles per hour (mph), in order to lock the pump/impeller and turbine members of the torque converter assembly. In this manner, slip is prevented at higher vehicle speeds.
Below this threshold speed, the TCC is disengaged to allow an increasing amount of slip to occur across the torque converter assembly. Coinciding with a fully-opened TCC at relatively low vehicle speeds, usually approximately 6 mph or less, a transition out of an active regenerative braking event occurs as the HEV is slowed to a standstill, or when the HEV is abruptly accelerated. When this occurs, the direction or sign of the total driveline torque, i.e., the sum of the engine torque and any transient electric motor torque supplied to the engine shaft from the motor/generator in its electric motor mode, transitions from a negative torque value to a neutral torque value or a positive torque value. Under certain circumstances, the transition or “let up” may produce a perceptible reduction in the vehicle deceleration rate.